International Journal of Applied Engineering Research ISSN 0973-4562 Volume 12, Number 1 (2017) pp. 11-21 © Research India Publications. http://www.ripublication.com 11 Influence of vinyl acetate-ethylene copolymer on early-age ettringite formation and behavior in OPC/CAC /hemihydrate gypsum binder system: A case of higher CAC content than OPC content in the binder Peter Tumwet Cherop 1 , Sammy Lewis Kiambi 2 , Eric Kiprotich Kosgey 3 1,2,3 Department of Chemical Engineering, Durban University of Technology, P.O Box 1334, Durban 4000, South Africa. 1 ORCID - 0000-0002-8991-1972 Abstract In this work, the effect of vinyl acetate-ethylene (VAE) copolymer on ettringite formation and behavior at early age in OPC/calcium aluminate cement/hemihydrate gypsum binder system (BS) was investigated. During the study the VAE dosage was 3 % by weight of the binder. The phase composition of the hydrated BS in air after 6 hours, 1, 3 and 7 days of hardening was investigated by XRD. The microstructure of the hydrated and hardened BS pastes were studied by scanning electron microscopy (SEM). Results from the study indicate that VAE speeds up the hydration of hemihydrate gypsum and slows down the ettringite formation. It was also observed that after 7 days of hardening, the presence of VAE in the BS appears to stabilize the ettringite. There is a change in the appearance of ettringite crystals in VAE-modified BS pastes, i.e. prismatic crystals turn into needle-like crystals, while the average crystals size decreases. Key words: Cement; Ettringite; Vinyl acetate-ethylene; X-Ray Diffraction; SEM INTRODUCTION Dry mix mortars are multicomponent systems that consist of mineral binders (calcium sulfates or ordinary Portland cement), , aggregates and additives such as superplasticizers, organic binders, retarders, accelerators, defoamers, cellulose ethers etc [1, 2]. The use of mineral binders regulates the setting time and strength development in mortars. For instance, in some self- levelling underlayments (SLU) or repair mortars fast setting and development of strength is required. To achieve this, calcium alumina cement (CAC) is used to supplement ordinary Portland cement (OPC).The major mineral phase in CAC is monocalcium aluminate (CA), which is very reactive when it gets in contact with water. The minor phases of CAC, which are less reactive, include CA2 and C12A7 [2, 3].The enhanced early ettringite formation leads to an accelerated hydration in OPC/CAC mixtures [2, 4-6]. The mortars in which shrinkage is compensated contain not only OPC and CAC, but also calcium sulfate binders such as anhydrite and hemihydrate gypsum. This kind of sulfate has an influence on the products formed during CAC hydration [7] .The use of anhydrite leads to formation of large quantities ettringite during hydration. Ettringite takes in 48.9 wt.% of water into its crystal structure leading to high consumption of water and consequently fast setting of mortars as a result of chemical desiccation [2]. The huge water uptake and development of ettringite needles result in expansion thereby facilitating shrinkage-compensation [2, 7]. In ternary binder systems made up of OPC/CAC and calcium sulfates, additives such as Seignette salt (K/Na tartrate) and lithium carbonate (Li2CO3) are included in order to control hydration kinetics so as to achieve a sufficient enough workability period and to provide high early strength [8-10]. Goetz-Neunhoeffer [11] came up with a model that explains the working mechanism of Li2CO3 and tartaric acid in CAC/hemihydrate mixtures. He found out that Li2CO3 selectively speeds up the hydration of CA and it does so by increasing its rate of dissolution and through formation of a lithium aluminum-layered double hydroxide compound. According to the model, in absence of Li + , a non-crystalline layer of aluminium hydroxo hydrate [Al(OH) x(H2O)y] is formed owing to low Al 3+ solubility under the slightly alkaline conditions. The [Al(OH)x(H2O)y] wraps the phases of CA and therefore acts as a diffusion barrier hence prevents it from being dissolved further. The use of Li2CO3 facilitates the conversion of Al 3+ into lithium aluminum hydroxide hydrate (LA2H10), which is precursor for ettringite formation. Therefore Li + catalyzes crystallization of ettringite [11]. Latex polymers are important components in most drymix mortars since they enhance adhesion and cohesion of fresh mortar on solid substrates and improve the brittleness and flexural strength of hardened mortar [12-15].These effects are achieved as a result of formation of latex films in the cement paste [16-19]. The most common latex polymers are copolymers made from vinyl acetate-ethylene (VAE), styrene/acrylates or styrene /butadiene [20]. A number of studies have been done about these materials [12, 21-23].Additionally, several studies have been done on the effect of latex polymers on hydration of OPC [22, 24-28]. Recently, Baueregger, et al. [2] conducted a study on the effect of latex polymers on hydration in the OPC/CAC/anhydrite binder system (BS).
Influence of vinyl acetate-ethylene copolymer on early-age ettringite formation and behavior in OPC/CAC /hemihydrate gypsum binder system: A case of higher CAC content than OPC content
May 05, 2023
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